Wear phenomena

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BK50A2200
Design Methodologies and
Applications of Machine
Element Design
Lecture 4
Wear Phenomena
D.Sc Harri Eskelinen
Wear
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Wear can be divided into SLIDING WEAR, which occurs
in the absence of hard particles, and ABRASIVE WEAR,
which occurs in their presence.
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Abrasive wear can be further subdivided into TWO-BODY
and THREE-BODY ABRASIVE WEAR.
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Under some conditions sliding wear can generate debris which
then causes further wear by abrasion; it must therefore always
be borne in mind that the boundary between different types of
wear is not a rigid one.
Two-body wear is caused by hard protuberances on the counter
face, while in three-body wear hard particles are free to roll and
slide between two, perhaps dissimilar, sliding surfaces.
A further type of wear involving hard particles is
EROSIVE WEAR, where hard particles carried in a gas or
liquid stream strike a surface.
Wear
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In materials science, wear is the erosion of
material from a solid surface by the action of
another solid. The study of the processes of
wear is part of the theory of tribology. There are
four principal wear processes:
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1.
2.
3.
4.
Adhesive wear
Abrasive wear
Fatigue wear or Surface fatigue
Tribochemical wear or Corrosive wear
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These four wear phenomena can affect
simultaneously but typically one of them is
dominant.
Wear can be defined as a process in which
interaction of surfaces of a solid with the
working environment results in the dimensional
loss of the solid, with or without loss of material.
The definition of wear does not include loss of
dimension from plastic deformation, although
wear has occurred despite no material removal.
Also this definition fails to include
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- impact wear where there is no sliding motion or
- cavitation or corrosion where counter body is a fluid.
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Wear environment includes different load types
such as
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- varying speed and temperatures,
different counter bodies
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unidirectional sliding,
reciprocating,
rolling,
impact
different loading conditions such as
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-
- solid,
- liquid,
- gas,
and different types of contact such as
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- single phase or multiphase, in which phases
involved can be liquid plus solid particles plus gas
bubbles .
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Wear along with other aging processes, such as, fatigue,
creep and fracture toughness cause progressive
degradation of materials with time leading to failure of
material at an advanced age.
The mechanism of wear is very complex and the
theoretical treatment without the use of rather sweeping
simplifications is not possible.
It should be understood that the real area of contact
between two solid surfaces compared with the apparent
area of contact is invariably very small, being limited to
points of contact between surface asperities.
The load applied to the surfaces will be transferred
through these points of contact and the localized forces
can be very large.
The material intrinsic surface properties such as
hardness, strength, ductility, work hardening etc. are
very important factors for wear resistance, but other
factors like surface finish, lubrication, load, speed,
corrosion, temperature and properties of the opposing
surface etc. are equally important.
Adhesive wear
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Adhesive wear is also known as scoring, galling, or
seizing.
It occurs when two solid surfaces slide over one another
under pressure. Surface projections, or asperities, are
plastically deformed and eventually welded together by
the high local pressure. As sliding continues, these
bonds are broken, producing cavities on the surface,
projections on the second surface, and frequently tiny,
abrasive particles, all of which contribute to future wear
of surfaces.
For adhesive wear to occur it is necessary for the
surfaces to be in intimate contact with each other.
Surfaces which are held apart by lubricating films, oxide
films etc. reduce the tendency for adhesion to occur.
A joint
Motion
A particle
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Excessive load, low speed and/or reductions in fluid viscosity can
reduce the oil film thickness to a point where metal-to-metal contact
occurs. Surface peeks are "cold welded" together and particles are
sheared off as surfaces move.
Abrasive wear
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When material is removed by contact with hard particles,
abrasive wear occurs. The particles either may be present at
the surface of a second material or may exist as loose
particles between two surfaces. Abrasive wear can be
measured as loss of mass by the Taber Abrasion Test
according to ISO 9352 or ASTM D 1044.
The abrasive wear mechanism is basically the same as
machining, grinding, polishing or lapping that we use for
shaping materials. Two body abrasive wear occurs when one
surface (usually harder than the second) cuts material away
from the second, although this mechanism very often
changes to three body abrasion as the wear debris then acts
as an abrasive between the two surfaces.
Abrasives can act as in grinding where the abrasive is fixed
relative to one surface or as in lapping where the abrasive
tumbles producing a series of indentations as opposed to a
scratch.
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Abrasive Wear Effects:
•
Dimensional changes
•
Leakage
•
Lower efficiency
•
Generated particles contribute more wear
Abrasive wear is a primary wear mechanism. Particles enter the
clearance space between two moving surfaces, and act like cutting
tools to remove material from the surfaces. The particle sizes
causing the most damage are those equal to and slightly larger than
the clearance space. To protect opposing surfaces from abrasive
wear, particles of approximately the operating clearance size range
must be removed.
Estimated hardness values:
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Pure aluminium
Pure copper
Medium carbon steels
Stainless steels (AISI 304)
Hardened steels
Chromium steels
Surface plating with Chromium
Chromium Carbide
Tungsten Carbide
Titanium Carbide
Diamond
15 HB
35 HB
120 HB
250 HB
650…700HB
700 HV
1000 HB
1200 HV
1400 HV
2400 HV
8000 HV
Suitable polymers for wearing conditions
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PEEK (Polyetheretherketon)
PES (Polyethersulfon)
PI (Polyimide )
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Low friction coefficient
High heat resistance (over 150 Celsius)
Low wearing rate
Possible to reinforce with different fibers
Tribochemical or Corrosive wear
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A tribological system consists of the
surfaces of two components that are in
moving contact with one another and
their surroundings. The type, progress
and extent of wear are determined by the
materials and finishes of the components,
any intermediate materials, surrounding
influences and operating conditions.
Tribological system
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1 Base object
2 Opponent body
3 Surrounding influences: Temperature,
relative humidity, pressure
4 Intermediate material: Oil, grease,
water, Particles, contaminants
5 Load
6 Motion
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Often referred to simply as “corrosion”, corrosive wear is
deterioration of useful properties in a material due to reactions with
its environment.
One form of high temperature corrosive (oxidative) wear can lead to
the formation of compacted oxide layer glazes, which under certain
circumstances reduces wear.
Erosion
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Wear due to mechanical interaction between that surface and a fluid, a
multicomponent fluid, or impinging liquid or solid particles
Cavitation Erosion
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A form of erosion causing material to wear by the action of vapour
bubbles in a very turbulent liquid.
Cavitation is the formation and collapse, within a liquid, of cavities or
bubbles that contain vapour or gas. Normally, cavitation originates from
changes in pressure in the liquid brought about by turbulent flow or by
vibration, but can also occur from changes in temperature (boiling).
Cavitation erosion occurs when bubbles or cavities collapse on or very
near the eroded surface. The mechanical shock induced by cavitation is
similar to that of liquid impingement erosion causing direct localized
damage of the surface or by inducing fatigue.
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Tribological sliding contact leads to a chemical reaction.
The reaction products influence the tribological
processes at the surface; for instance, pairs of
components with narrow tolerances can jam.
In general, tribochemical wear increases with rising
temperature. A frequent cause of tribo-chemical wear is
oxidation.
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Erosion is caused by a gas or a liquid which may or may not carry
entrained solid particles, impinging on a surface. When the angle of
impingement is small, the wear produced is closely analogous to
abrasion. When the angle of impingement is normal to the surface,
material is displaced by plastic flow or is dislodged by brittle failure.
Fatigue wear or Surface fatigue
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Surface fatigue is a process by which the surface
of a material is weakened by cyclic loading,
which is one type of general material fatigue.
Fretting Wear
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Fretting is a small amplitude oscillatory motion,
usually tangential, between two solid surfaces in
contact. Fretting wear occurs when repeated loading
and unloading causes cyclic stresses which induce
surface or subsurface break-up and loss of material.
Vibration is a common cause of fretting wear.
Calculation of Wear Rate
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A common used equation to compute the wear
rate is (Archard,1953).
Vi =ki × F × s
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where F is the normal load, s the sliding
distance, Vi the wear volume and ki the specific
wear rate coefficient. Index i identifies the
surface considered.
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The k-value is given in m3/Nm or m2/N.
From design view the wear displacement h is
more convenient than the wear volume V.
With hi =Vi /A and the contact pressure p=F/A,
where A is the area subjected to wear, we get:
hi =ki × p × s
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The sliding distance s can finally be replaced by
s=v × t where v is the mean value for the slide
rate and t the running time.
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Because the k-value depends just like the
friction coefficient on a lot of parameters this
factor is to be find experimentally.
In standard wear test results, the loss of
material during wear is expressed in terms of
volume. If wear displacement h is not used, the
volume loss gives a truer picture than weight
loss particularly when comparing wear
resistance properties of materials with large
variations in density.
For engineering components the working life is
over when the dimensional losses exceed the
specified tolerance limits.
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Advanced equation: V =Ki×SC2 × RC3
V
S
R
Ki , C2,C3
=
=
=
=
volume of worn-off material
surface stress at the contact point
number of loading cycles
coefficient depending of material pair and surface
properties
This advanced equation is developed to take into
account also the affects off the peeks in surface
profile and affects off repeated loading cycles
In some simple load cases also the friction
coefficient can be used to estimate the wear
rate
TYPES OF LUBRICATION
FULL-FILM LUBRICATION
MIXED-FILM LUBRICATION
BOUNDARY LUBRICATION
• Hydrostatic lubrication
• surfaces may occasionally
contact with each other
though in usual cases
lubrication is sufficient
• surfaces suffer from contacts
with each other due to
- over loading
- lack of lubricant
- surface roughness
- impurities between the
surfaces
- elevated hydrostatic pressure caused with
a pump, geometry etc.
• Hydrodynamic lubrication (HD)
- a small gap between the surface forms
the pocket for the lubricant due to right
relative speed and size of the gap
• Elastohydrodynamic lubrication (EHD)
- if the geometry is nonconforming
(like in gears ) high speed and enough load
is required to keep the lubricant between
the surfaces
• The thickness of the film and the viscosity
are in key-role!
Examples
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Case 1. Affects of wear in a scotch yoke mechanism
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Case 1. Affects of wear in a cam-follower mechanism
SUMMARY : WEAR PHENOMENA
ABRASIVE WEAR
ADHESIVE WEAR
• hard particles destroy the
softer surface
• surfaces tend to get stuck to
each other
Theory
Application
DIFFERENT CHAINS OF WEAR
PHENOMENA
Theory
Application
Adhesive Tribochemical
Abrasive
Adhesive
Abrasive
TRIBOCHEMICAL WEAR
• chemical reactions take
place on the surfaces
Theory
Application
“corrosion wear”
Fatique
Abrasive
Abrasive
Tribochemical
Adhesive
Application
“scoring or scuffing or galling”
FATIQUE WEAR
• cyclic load affects under the
surface layer
Theory
Application
“surface fatigue or pitting”
Exercise 4
Exercise 4A
 Write brief definitions for the following wear mechanisms (use only one sentence
and describe each item with your “own words”) :
 Adhesive wear
 Abrasive wear
 Tribochemical wear
 Surface fatigue
Exercise 4B
 Try to find different types of applications, in which different wear mechanisms
affect as sequential phenomena (eg. adhesive  abrasive).
Exercise 4C
 Select suitable material types for a rolling contact surface, which should
withstand abrasive wearing conditions caused by fine sand dust (hardness about
650 HV).
Exercise 4D
 Select suitable surface coatings (or alloying) for a component, which should
prevent adhesive wear in case where the construction material is:
 Steel
 Aluminium
 Nickel